In general, a cold rolled steel sheet, a galvanized steel sheet, and a galvannealed steel sheet which are used as an exterior panel material and the like of automobiles are manufactured by heating a steel strip serving as a raw material with a continuous annealing furnace, cooling the resultant, or, after the heat treatment, guiding the resultant to a molten zinc bath for hot dip galvanizing, and controlling the adhesion amount to a given amount, or, thereafter, further guiding the resultant to an galvannealing furnace by gas heating, induction heating, etc., for galvannealing treatment in which iron is thermally diffused into a plating layer, and then subjecting the resultant to temper rolling.
The continuous annealing furnace used for manufacturing a steel sheet containing a heating zone and a soaking zone in which a steel strip is heated to a predetermined temperature for annealing treatment and a cooling zone in which the high temperature steel strip is cooled to room temperature or a given temperature. In some cases, the rapid cooling zone contains, in addition to the case of containing a usual cooling zone alone, a rapid cooling zone and a slow cooling zone or a rapid cooling zone and an overaging treatment zone or the like.
In contrast, a steel sheet for automobiles has been becoming increasingly wide with an increase in size of automobiles. From the viewpoint of securing safety, an increase in strength of a steel sheet for automobiles has been advanced. Moreover, from the viewpoint of protecting the global environment, a steel sheet for automobiles has been becoming increasingly reduced in thickness to reduce the weight of a car body. As a result, in a continuous annealing furnace, the annealing temperature increases with the employment of a high strength material. The increase in annealing temperature reduces high temperature strength of a steel strip, resulting in a tendency that threading properties of the steel sheet deteriorate. Furthermore, due to the abovementioned broadening and thickness reduction of a steel strip, it is becoming difficult to maintain stable operation of a continuous annealing furnace with a conventional continuous annealing technique.
In recent years, by positively utilizing a material control technique of rapidly cooling a high temperature steel strip in a continuous annealing furnace, development of a product having both formability and strength has been vigorously performed. As an example thereof, a bake-hardening steel sheet (a so-called “BH steel sheet”) used for car bodies is mentioned.
As a rapid cooling technique applied to manufacturing of the steel sheet, techniques, such as a gas jet cooling system including spraying cooling gas, which is obtained by cooling atmospheric gas with a heat exchanger, onto a steel strip as high-speed gas jet flow and a roll cooling system including pouring a cooling medium into a roll to cool the roll, and pressing the cooled roll against a steel strip to cool the steel strip, are known.
Among the above, the gas jet cooling system has advantages that the appearance and shape of a steel strip after cooling are relatively favorable and that cooling equipment are also relatively inexpensive but has a disadvantage that a cooling rate is low, compared with other cooling methods. Then, to compensate for the disadvantage, the flow rate of coolant gas has been increased or the distance between a gas jet nozzle and a steel strip has been reduced to increase the heat transfer rate.
However, increasing a cooling gas jet flow rate or reducing the distance between a gas jet nozzle and a steel strip are effective for increasing the heat transfer rate, but there arises a problem that, when cooling gas hits the steel strip surface, a steel strip edge is excessively cooled, promoting temperature non-uniformity along the widthwise or transverse direction of the steel strip. Furthermore, in manufacturing a bake-hardening steel sheet, there arises a problem that a defect referred to as a stretcher strain or a wrinkle pattern may occur due to the temperature non-uniformity along the widthwise or transverse direction of a steel strip. Furthermore, it has been desired to control the temperature and the cooling rate in a continuous annealing furnace with higher precision with tightening of quality requirements in a steel sheet for automobiles.
Then, some techniques of reducing the temperature non-uniformity along the widthwise or transverse direction of a steel strip have been proposed. For example, Japanese Patent Application Laid-Open No. 06-179955 discloses a technique of disposing an induction heater at a forward part or a rear part of a rapid cooling zone of an annealing furnace having a deoxidation furnace. Japanese Patent Application Laid-Open No. 2004-339553 discloses a technique of disposing an induction heater at a rear part of a cooling furnace for induction heating. Furthermore, Japanese Patent Application Laid-Open No. 2005-113244 discloses a technique of disposing, at a stage prior to a molten zinc bath, an edge heating roll having a plurality of induction heating coils for heating the end of a steel sheet.
However, according to the method disclosed in Japanese Patent Application Laid-Open No. 06-179955, an induction heater is disposed at a rear part of a cooling zone of a horizontal type furnace and, according to the method disclosed in Japanese Patent Application Laid-Open No. 2004-339553, an induction heater is disposed at a rear part of a cooling zone of a vertical type furnace. Thus, in the case of a steel strip having a wide width and a thin thickness, catenary or cross bow of a steel strip is noticeable and when the shape of a steel strip is bad, contact between a heating coil of an induction heater and a steel strip easily occurs. Therefore, there is a possibility that troubles, such as a spark or water leakage from a heating coil cooling unit due to the spark, occur or a steel strip brakes. According to the method disclosed in Japanese Patent Application Laid-Open No. 2005-113244, the winding angle of an edge heating roll described in Examples is about 120°. When a steel strip has a wide width, a thin thickness, and a bad shape, contact between a steel strip and a roll becomes insufficient, resulting in a failure of sufficiently obtaining the effects of an edge heating roller.
According to a method involving directly heating a steel strip from a heating coil with an induction heater disposed in an annealing furnace, repairing of the induction heating coil requires opening the furnace, resulting in that the furnace needs to stop over a long time. Furthermore, in high-rate gas jet cooling equipment having a heat transfer coefficient (α) per side of a steel strip of 170 W/m2/° C. or higher, cooling gas impinging on the surface of a steel strip flows out of a cooling zone to develop temperature non-uniformity along the widthwise or transverse direction of the steel strip in a hearth roll located at a position nearest to an inlet or an outlet of a rapid cooling zone, and promoting excessive cooling at an edge part of the steel strip behind the rapid cooling zone or temperature non-uniformity along the widthwise or transverse direction of the steel strip. The temperature non-uniformity along the widthwise or transverse direction of the steel strip results in non-uniformity in a dissolved carbon content along the widthwise or transverse direction of the steel strip, and further seriously affect the development of a stretcher strain or a wrinkle pattern of a bake-hardening steel sheet.
Thus, it could be helpful to provide continuous annealing equipment capable of eliminating temperature non-uniformity along the widthwise or transverse direction of a steel strip at an exit side of a rapid cooling zone having gas jet cooling equipment and preventing the development of a stretcher strain or a wrinkle pattern of a bake-hardening steel sheet.